Conventionally, in order to illuminate the key parts of a portable electronic device (hereinafter referred to as a portable device), a structure in which LEDs are mounted and point light emission is diffused by the key tops is used. FIG. 10 shows a first example of the configuration of the key module of a conventional mobile device. In this example, domes 24 are disposed below corresponding key tops 26 formed on a base plate 22. Each of the domes 24, when depressed by the corresponding key tops 26, deforms, electrically connects to a corresponding contact, and also imparts a predetermined clicking sensation. This dome 24 is fixed on the base plate more securely by its being covered by a dome sheet 25 from above. In addition, each LED module 23 is disposed between the domes 24 on the base plate 22. Disposed above the LED module 23 is a light incident part 26a for the corresponding key top 26. When the LED module 23 emits light, the light exiting from this LED module 23 is incident to the light incident part 26a and is diffused by the key top 26.
However, in this configuration, the light source is the LED module 23, which is a point light source, and the uniformity of luminance is low, leading to inconveniences, such as poor visibility. Such a configuration also suffers from the problem that an increase in the number of LED modules 23 increases costs and power consumption.
To overcome such problems, a configuration has been proposed, as in an example shown in FIG. 11, in which a light guide plate 31 is disposed between a dome sheet 35 and a key top 36, incident light from an LED module 33 enters from a side of the guide light plate 31, and exits from the surface of the light guide plate 33 as surface light. This makes it possible to reduce the number of LED modules 33, improve the uniformity of luminance, and facilitate the visibility of characters on each key top 36.
However, a conventional light guide plate 31 is formed by molding acrylic resin or polycarbonate, which makes it difficult to reduce the thickness of the light guide plate. A thicker light guide plate 31 makes it more difficult for the plate 31 to follow the recesses and projections of the dome sheet 35, with the result that clicking sensation is not obtained when the keys are operated. Accordingly, this example has a structure in which openings 31a are formed in parts of the light guide plate 31, which parts are near and above the domes 34, so that the key tops 36 come into contact with the dome sheet 35 directly. In such a structure, a large quantity of light propagating horizontally in FIG. 11 exits toward the key tops 35 from the end faces that define the openings 31a. Consequently, light does not reach the trailing end of the light guide plate 31, making it difficult to maintain uniformity of luminance.
Additionally, in the example shown in FIG. 11, assembly is inefficient and requires extra space, and incurs additional processing costs such as those for boring a hole in the light guide plate 31. This increases costs and hinders attempts to decrease thickness. A technique for decreasing thickness by employing a polycarbonate film as a light guide plate 31 has been developed. In this case, however, clicking sensation cannot be obtained due to the film's hardness; and improving clicking sensation by making the film thinner results in insufficient light incident to the film and hence a luminance decrease and loss of uniform luminance.
On the other hand, there is a technique for employing a flexible silicone rubber sheet for use as a light guide plate. The silicone rubber produces excellent clicking sensation compared to polycarbonate, but suffers from decreasing durability if the silicone rubber film is made thinner.
Further, there is a technique that uses a polyurethane sheet as a light guide plate. The polyurethane sheet is superior in clicking sensation to polycarbonate film of the same thickness and in durability to the silicone rubber sheet. Depending on a processing method, polyurethane may be categorized as thermosetting or thermoplastic type. Polyurethane for use as a light guide plate is of thermoplastic type that can easily be molded into a sheet. Various materials may be added to the thermoplastic polyurethane in order to improve its moldability. However, this makes the thermoplastic polyurethane inferior in oil resistance to thermosetting polyurethane; moreover, thermoplastic polyurethane shrinks or deforms due to heat, making it difficult to use where heat resistance is required. Such a characteristic of thermoplastic polyurethane does not favor its use in an optical film that requires reliability, as in the present invention.
Patent Document 1: Japanese Patent Application Laid-Open 2007-53063
Patent Document 2: Japanese Patent Application Laid-Open 2007-87749
Patent Document 3: Japanese Patent Application Laid-Open 2001-167655
Patent Document 4: Japanese Patent Application Laid-Open 2007-324100
Patent Document 5: Japanese Patent Application Laid-Open 2008-243655